Quantum dot lasers on silicon substrates through evanescent coupling

Objective The integration of on-chip silicon-based light sources has been a critical challenge in addressing the absence of light sources in silicon-based optoelectronic chips. This integration is essential for enhancing the power output of light sources on silicon-based optoelectronic chips, thereby increasing the chip's speed and capacity. However, traditional integration methods such as end-face coupling suffer from low efficiency due to issues like surface roughness and alignment difficulties, which are difficult to mitigate. Therefore, this paper proposes a method for light source integration using evanescent coupling, which can improve light source coupling efficiency and increase the output power of silicon-based on-chip light sources. Furthermore, traditional silicon-based light source integration typically employs quantum well lasers, whereas this study designs an appropriate quantum dot laser, which offers advantages such as lower threshold current and more stable temperature tolerance. These benefits enhance the overall coupling efficiency and output power of the silicon-based on-chip quantum dot laser. Methods Using Lumerical simulation software, we optimized the evanescent coupling waveguide structure and the Bragg grating waveguide structure for an O-band silicon-based on-chip light source. At a wavelength of 1.31 μm, the tapered waveguide structure designed for evanescent coupling achieved a coupling efficiency of over 98% for widths of 0.68 μm, 0.7 μm, and 0.75 μm (Fig.4). The study primarily analyzed the impact of length on coupling efficiency. Additionally, we designed two segments of Bragg gratings to form a resonant cavity, with the paper focusing on the effects of duty cycle, etch depth, and grating length on reflectivity (Fig.7). This structure can achieve reflectivities of 40% and 90% at the three specified waveguide widths, allowing for the amplification and selection of light at a wavelength of 1.31 μm (Fig.8). Results and Discussions Through waveguide mode analysis, a waveguide layer thickness of 220 nm was selected, with waveguide widths of 0.68 μm, 0.7 μm, and 0.75 μm. For these three waveguide widths at a wavelength of 1.31 μm, we designed a tapered waveguide coupler structure with a length of 32 μm, where the width transitions from the waveguide width to 2 μm and then narrows back to the original waveguide width, achieving a coupling efficiency of over 98%. A Bragg grating structure was designed with an etch depth of 100 nm and a duty cycle of 0.75 on both sides, with lengths of 110 μm and 240 μm, achieving reflectivities of 40% and 90%, respectively. These two Bragg gratings form a resonant cavity that amplifies and selects light at a wavelength of 1.31 μm. Conclusions This study focuses on silicon-based light sources, with particular emphasis on coupling efficiency, wavelength selection, and integration. Specifically, we designed a 142 μm tapered evanescent coupling structure, achieving a coupling efficiency of over 98%. The Bragg gratings, serving as both mirrors and resonators, are positioned on both sides of the waveguide, with lengths of 110 μm and 240 μm, respectively. These gratings selectively amplify light at a wavelength of 1.31 μm by facilitating oscillation within the cavity. This design not only enhances the emission efficiency of silicon-based light sources but also reduces manufacturing costs and is highly compatible with existing silicon-based photonic manufacturing processes. Compared to previous studies, our work achieves efficient light source integration and single-sided narrowband laser output by optimizing the coupling structure and mirrors. © 2024 Chinese Society of Astronautics. All rights reserved.